Try going back to Vacuum Advance...Off Intake???

[Guest]

How about if I try going to vacuum advance. My Dizzy is a fresh rebuild with one bellow. Can I simply hook a small hose from the bellow directly to the small (currently plugged) inlet on the intake manifold, or will I suck the guts out of my dizzy? I currently am running 8* BTDC with no vacuum. Real an article that talked about the importance of vacuum advance on a street engine and how centrifugal advance only really helps on a full race setup.

 

[Brosky]

Re: Try going back to Vacuum Advance...Off Intake?

Bill, if that were true, then why do the later cars have a vacuum retard instead of an advance? I think that something is sticking or your coil is bad. Make sure that your advance is working it's way up to about 34-36 degrees total in a smooth fashion, both up and down the scale as engine speed increases and decreases.

 

[StagByTriumph]

Re: Try going back to Vacuum Advance...Off Intake?

Depending on the year of ZS carb in the USA, the vacuum may indeed be a vacuum retard for emissions, and that vacuum signal hose is connected to the bottom of one of the carburetors (175 CDSE).
Provided of course it is stock.

According to the 1973 Emsiision tuning training booklet, Vacuum retard will not occur until there was more than 2.5 in Hg, full retard was to 7 degrees at 10 inches Hg.

Seems like I just recently saw a link somewhere for the 73 emissions tuning training booklet too ... now where was that ...

Oh well, send me $15 and I'll put it on a CD.

 

[martx-5]

I see no reason why you can't hook it up directly to the intake manifold. For those of you interested, here is an article explaining how the vacuum advance works. Also, the vacuum retard is strictly an emissions device to cut down on the NOx. It's just another Band-Aid approach to solving a problem that wound up hindering engine performance.

 

[dklawson]

Most (but not all) Lucas dizzys were designed to connect to the carb (ported vacuum). A lot of non-LBC cars do have vacuum connections to the intake manifold (manifold vacuum). IF you hook a dizzy to the intake manifold and that dizzy's vacuum canister was designed for ported vacuum, you will get a huge amount of advance at idle. You'll know immediately that things have changed the second you hook the tubing to the intake manifold. Typically the RPM go up, WAY up. I'm not saying what you're proposing won't work, I'm just saying it may not achieve what you're expecting.

As mentioned above, vacuum retard appeared in the mid- to late 1970s for emissions controls.

 

[bobh]

There is an article that explains timing on a TR6. It discusses the earlier TR6s with vacuum advance and retard. I suspect your distributor has vacuum retard only. The vaccuum ports on the carbs are purpose built. One for retard and one for advance. One is sheilded by the throttle butterfly at idle so it has no effect until the throttle is opened....
You should read the article before deciding how to hook up your vacuum module.
https://www.tr6web.com/Documents/tr6/advretard.html

BUB

 

[martx-5]

Most (but not all) Lucas dizzys were designed to connect to the carb (ported vacuum). A lot of non-LBC cars do have vacuum connections to the intake manifold (manifold vacuum). IF you hook a dizzy to the intake manifold and that dizzy's vacuum canister was designed for ported vacuum, you will get a huge amount of advance at idle.

OK, I'm not understanding something here, so please explain what the difference is in PORTED vacuum and INTAKE vacuum.

Edit: Never mind, I just looked at the SU carb on my TR3, and I notice that the vacuum port is on the air cleaner side of the throttle plate until you just crack it open. Ergo...no appreciable vacuum advance at idle.

That would mean you can't just hook the vac advance to the intake. I guess that's why nobody uses the vac advance on the DCOE carbs.

 

[dklawson]

Just for clarity...

Ported Vacuum = hooked to the carb body, typically getting vacuum from the venturi. The vacuum will be highest at wide open throttle when the most air is pulled through carb. (Which sounds like what you observed on your TR3).

Manifold Vacuum = hooked to the intake manifold (between carb and head). The vacuum will be strongest at idle and lowest at wide open throttle.